The present invention relates to a miniature electric motor for use in optical precision equipment such as compact camera, audio visual equipment such as a headphone stereo cassette tape recorder, and automotive electronic equipment. In particular, the invention relates to a miniature electric motor provided with a preassembled commutator.
Miniature motors have been widely used in various fields including the above-mentioned equipment and have been highly miniaturized to reduce their size and weight as well as their height.
In the motors, permanent magnets are mounted within an interior of a casing, and a rotor is disposed inside the permanent magnets. A rotary shaft of the rotor is rotatably supported by bearings mounted on the casing. A commutator is provided around the rotary shaft. Brushes mounted on the casing are slidingly engaged with the commutator.
Commutators which are components of the motors are classified into a mold type such that the entire structure is integrally formed of thermosetting resin and a preassembled type such that thermoplastics resin is used at dielectric parts.
The mold type and the preassembled type commutators are preferably selected depending upon revolving speed of the rotors or the circumstances where the motors are to be used. In general, it is preferable to use the preassembled type commutator for the case of the low revolving speed and to use the mold type commutator, which can withstand heat, for the case of the high revolving speed or a large size motor.
In the case where the revolving speed of the motor is low, since the current would be small to generate a small amount of heat and a low mechanical strength would suffice, a relatively thin metal plate would be used as commutator pieces of the preassembled commutator.
On the other hand, of the mold type commutator, since a relatively thick plate is used as commutator pieces and thermosetting rein is used for the dielectric parts, even if a large amount of current is caused to flow to generate a large amount of heat, a sufficient heat resistivity is insured. Accordingly, this type of the commutator is suitable for relatively large motors.
In contrast thereto, according to the preassembled commutator, there would be a fear that the thermoplastics resin or the commutator pieces would be molten by heat if a large amount of current would be caused to flow. It is necessary to reduce the amount of the heat. Accordingly, this type of the commutator is suitable for relatively small motors. Also, since the preassembled type commutator is small in size, a large mechanical strength would not be required.
For instance, a preassembled commutator is well known which is composed of a sleeve having a flanged portion at one end of a cylindrical portion, a plurality of commutator pieces made of metal and mounted around the sleeve, and a ring mounted around outer peripheries of the commutator pieces.
In such a commutator, means for fixing the plurality of commutator pieces to the sleeve is formed depending upon the pressure applied from the ring only. Accordingly, there is a fear that the commutator pieces would be offset in the circumferential direction when wirings of an armature are connected to terminal portions of the commutator pieces.
Also, one terminal portion is formed on each commutator piece and proximal portions of commutator pieces are depressed by the ring only. Accordingly, the mechanical strength of the commutator piece itself is low, and the fastening strength of the commutator piece to the sleeve is low, too. As a result, it would be difficult to make the outer circumferential surface of the plurality of commutator pieces into a true cylindrical shape.
Also, a distal end portion of each commutator piece would be raised radially outwardly, so that the true cylindrical shape of the outer circumferential surface would not be attained. The smaller the commutator, the more the assembling work would become difficult. Thus, the degradation of the true circle property of the outer circumferential surface would frequently be encountered.